The present invention generally relates to measuring devices and, more particularly, to dual-pulse output measuring devices for metering fluid flow.
Repeated measurement and correction of fluid flow is important to the proper operation of many industries. For example, in the petroleum industry, e.g., natural gas, gasoline, etc, cumulative measurements are required at many stages during production, transportation, refining, and selling of the product. Such measurements form the basis for royalties, physical and custody transfer accounting, and provide means for stock and loss control.
The measurement of such fluid flow is typically performed by an electrical or electronic device such as a turbine meter or the like. The turbine meter typically includes a turbine wheel with angled blades which are adapted to rotate as flowing fluid imparts force thereto. The turbine blades typically rotate past a pair of spaced sensors or transducers with the signals or pulses generated by the transducers being used to calculate a value indicative of fluid flow.
Such measuring devices are therefore dependent on the received pulses being accurate. However, noise or transients in the electrical transmission lines can sometimes result in false pulses, and broken or malfunctioning sensors or turbines can result in missed pulses. Other sources of errors in the pulse count output can result from such things as electromagnetic interference, power supply variations and or interruptions, inadequate signal level as a result of line loss, common mode noise induced in cabling, series mode noise induced in cabling, noise introduced from ground loop problems, excessive gain in frequency response of the system elements, spurious signals induced from other meters sharing the same multicore cable, short circuits or open circuits of conductor pairs, short circuits of either conductor to ground or shield, bad connections, temperature variations and extremes, vibration shock, and adverse environmental conditions.
Given the importance of accurate fluid metering, and the widespread use of such dual-output metering devices in the global marketplace, governing standards for the operation of such devices have been promulgated. The International Organization for Standardization (ISO) and the American Petroleum Institute (API) both advocate a five level standard establishing guidelines for insuring the fidelity and security of pulse data transmission systems (including the aforementioned dual turbine meters). The standard levels range from a least stringent level E, wherein accuracy is achieved solely by correctly installing apparatus of good quality, to a most stringent level A, wherein errors in the pulse count are not only detected, but corrected as well.
In one known device, a timer is used to determine if the pulses from both channels arrive simultaneously. If the pulses do arrive simultaneously, this is interpreted as common mode noise and both pulses are disregarded, even though one pulse may be valid. No correction for errors in the pulse count is provided.
In accordance with one aspect of the present invention, a dual pulse error detection and correction system is provided which includes first and second sensors, a memory, and a processor. The first and second sensors are adapted to generate first and second pulses related by a phase difference, and the processor is adapted to receive the first and second pulses from the sensors, calculate a current phase difference, compare the current phase difference to a desired phase difference stored in the memory, and generate an error signal if the current phase difference is not equal to the desired phase difference.
In accordance with other aspects of the invention, the processor is adapted to generate a corrected pulse count output. The corrected pulse count output accounts for extra pulses as well as missed pulses.
In accordance with another aspect of the present invention, a method of detecting and correcting pulse count errors of a dual pulse output related by a phase difference is provided which comprises the steps of receiving first and second pulses from a dual pulse output, calculating a phase difference between the first and second pulses, comparing the calculated phase difference to a desired phase difference, and generating an error signal if the calculated phase difference is not equal to the desired phase difference.
In accordance with other aspects of the invention, the invention further includes the step of calculating the desired phase difference before the receiving step, with the desired phase difference being determined based on a sampling of first and second pulses. Alternatively, the desired phase difference may be calculated based on an average phase difference.
These and other aspects and features of the present invention will become more apparent from the following detailed description when taken into conjunction with the accompanying drawings.